Modern internal combustion engines are featured with various exhaust after treatment devices to reduce the toxicity of emissions from the engine. Components typically used for treating the exhaust gas include:                a catalytic converter to break down gaseous pollutants in the exhaust gas;        a particulate filter (or soot filter) to remove the fine, solid particles in the exhaust gas (especially in diesel engines).        
As it is well known, exhaust gas treatment in diesel engines (operating with excess air) is nowadays carried out by means of an oxidation-type catalytic converter (also called Diesel Oxidation Catalyst or DOC). The role of the DOC is to break down pollutants in the exhaust stream into less harmful component.
The particulate filter (typically referred to as Diesel Particulate Filter in diesel engines—DPF), in turn, is designed to remove diesel particulate matter or soot from the exhaust gas. While such devices can attain great efficiency rates, they require a regular monitoring of their operating status and periodical cleaning.
The emission legislations in the US and Europe have introduced the need for the application of DPFs. In this connection, in order to fulfill future on-board diagnostic legislations (OBD), which require more stringent requirements on monitoring the particulate filter, it is necessary to detect the soot amount released by the DPF.
In this connection, Thorsten Ochs, et al., in “Particulate Matter Sensor for On Board Diagnostics (OBD) of Diesel Particulate Filters (DPF)”, SAE International, 2010-01-0307 (December 2010), pages 73 to 81, describe an OBD concept algorithm for monitoring a DPF using a resistive-type soot sensor directly placed downstream of the DPF.
Such resistive soot sensor is based on a multi-layer ceramic technology and comprises inter-digitated electrodes with an initially infinite electrical resistance. During sensor operation soot particles are collected onto the sensor and form conductive paths between the electrodes, giving rise to a current dependent on the collected soot mass. The accumulated soot particles are eliminated by burning during a regeneration phase, before a new measuring cycle starts.
The signal of interest, which is representative of the soot flow in the exhaust, is actually the time between the start of sensor operation (following a regeneration) and the reaching of a predetermined current threshold, which is referred to as the “response time” of the sensor. Hence, in practice, the response time correlates with the soot flow in the exhaust gas and has been used for OBD diagnostic.
The DPF OBD concept algorithm proposed by Ochs et al. relies on a limit DPF model, i.e. a model representing a DPF in the least acceptable operating condition. A model-based expected response time is calculated based on a simulated engine-out soot mass flow and taking into account the limit DPF model. The DPF OBD concept algorithm then compares the expected response time with the measured response time and can thereon draw conclusions about the operating status of the DPF. If the measured response time is lower than the predicted response time, the DPF is indicated as faulty.